Many modern communication systems are assembled from a number of smaller subsystems or stations that are geographically spaced apart from each other, but are arranged to work together. One such system is a paging system that typically comprises a paging terminal, a paging system controller, and a number of transmitter units, called paging stations, that are located over a wide geographic paging service area. The paging terminal is connected to the publicly switched telephone network and receives incoming calls to the system subscribers. In response to a call, the paging terminal formulates a page for the subscriber and forwards the page to the paging stations through the paging system controller.
The paging stations, upon receipt of the page, broadcast it over the air with their associated transmitting equipment. The subscriber's pager, which is a small receiver, picks up the broadcasts and, by the actuation of a display, generation of an audio tone, or generation of a tactile vibration, notifies the subscriber that the he is being paged. Other types of multistation systems include cellular two-way systems and data acquisition systems.
To ensure that multistation communications and measuring systems function properly, each station typically includes a clock that must be coordinated with the clocks of the other stations. In other words, each of the clocks must, at the same moment, indicate a time that is related to one another. For example, one paging system is arranged so that the paging system controller collects a number of pages, bundles them together in a packet, and then forwards the packet to the paging stations along with an instruction indicating when the packet should be broadcast. The paging stations then broadcast the packet of pages at the time indicated in the instruction. As long as all the stations broadcast the packet at exactly the same time, pager receivers carried by system subscribers who are in areas where paging signals from two or more stations that are about the same distance from the paging receivers will receive a signal that the pagers' circuitry can readily process. However, if the pages are broadcast at different times and/or received at different times, the pagers will receive multiple, overlapping signals that cannot be processed. As a result, when a subscriber carries a pager into one of these signal overlap zones, it may become operative. In order to avoid this undesirable result, all of the paging stations should have clocks that indicate the same time so that each station transmits the same packet of pages at the same time.
To date, it has proved difficult to economically provide a set of spaced apart locations, such as paging stations, with clocks that are all in synchronization. Although individual stations can be provided with very accurate clocks, such as atomic clocks, these clocks are very expensive. Furthermore, it is typically necessary that the coordination of these clocks be performed by a technician that visits the clock site on an all too frequent basis. The expenses associated with having personnel make such visits often means that such coordination occurs at a less than optimal frequency.
Other attempts at providing a multiclock coordination system have involved providing a master unit that generates a continuous reference signal used by each of the geographically spaced apart clocks to regulate their rate. Typically, the reference signal is some type of periodic signal and the clocks employ phase-locked loop subcircuits to regulate the advancement of clocks. A disadvantage of these systems is that only clock rate information is broadcast from the reference. Another method, such as monitoring the output of the paging stations with one or more radio recievers, is required to periodically set the clocks relative to each other in order to allow simultaneous broadcast of pages . These systems tend to be complex, fault sensitive, and wasteful of the radio frequency upon which pages are broadcast.